Inflammation contributes to axon reflex vasodilatation evoked by iontophoresis of an alpha-1 adrenoceptor agonist

Iontophoresis of α1-adrenoceptor agonists in the human forearm evoke axon reflex vasodilatation, possibly due to an accumulation of inflammatory agents at the site of iontophoresis. To investigate this possibility, skin sites in the forearm of healthy participants were treated with an anti-inflammatory gel containing ibuprofen 5% before the iontophoresis of the α1-adrenoceptor agonist phenylephrine (350 μA for 3 min). Red cell flux was measured with laser Doppler flowmetry at the site of iontophoresis and 8 mm away in the region of axon reflex vasodilatation. In additional experiments, skin sites were treated with the vasodilator sodium nitroprusside (to counteract vasoconstriction and disperse inflammatory mediators produced during the iontophoresis of phenylephrine); local anaesthetic agent (to determine whether the axon reflex to phenylephrine was neurally-mediated); or the α2-adrenoceptor agonist clonidine (to investigate the specificity of the adrenergic axon reflex). Phenylephrine evoked marked vasodilatation 8 mm from the site of iontophoresis whereas clonidine and saline-control did not (mean flux increase ± S.E. 485 ± 132% for phenylephrine; 44 ± 24% for clonidine; 39 ± 19% for saline-control; p < 0.05 for phenylephrine versus control). Axon reflex vasodilatation to phenylephrine was unaffected by variations in blood flow at the site of phenylephrine iontophoresis, but was reduced by ibuprofen pretreatment and abolished by local anaesthetic pretreatment. These findings suggest that prostaglandin synthesis at the site of iontophoresis contributes to but does not account entirely for axon reflex vasodilatation to phenylephrine. Alpha-1 adrenoceptor mediation of axon reflexes could play a role in aberrant sensory-sympathetic coupling in neuro-inflammatory diseases.